Gray out confirm button on pattern/pin/password set dialog

Setting a pin/pattern/password can take a second. Gray out the confirm
button to avoid multiple submissions.

Note that this requires async tasks so that the button is shown as gray
in a timely way.

Also don't add another stage - this causes a11y to repeat the title.

Bug: 22882174
Change-Id: Ib8047fde9e12afa25e82ebfa3a1e799a4b7043f2